smoke: prove the forgejo-runner executes a job

.forgejo/workflows/smoke.yml

@@ -0,0 +1,16 @@
+# Smoke workflow pushed by smoke-runner.sh into a throwaway ci-smoke repo to
+# prove the forgejo-runner actually EXECUTES a job (not just "registered").
+# runs-on: docker -> the job runs as a CONFINED container INSIDE the isolated
+# dind daemon (the act image), which also exercises the dind + job-container path
+# the real CI uses. We deliberately do NOT use a `host` label (that would run the
+# step in the runner container, which holds the dind admin certs).
+name: smoke
+on: [push]
+jobs:
+  smoke:
+    runs-on: docker
+    steps:
+      - run: |
+          echo "forgejo-runner smoke OK (docker backend / dind)"
+          echo "container: $(uname -srm)"
+          echo "date: $(date -u +%FT%TZ)"